Marshal Edward Mars was aboard Oceanic Flight 815 returning to Los Angeles after he had captured the fugitive Kate Austen. Edward Mars was employed as a US Marshal, and had been tracking Kate for several years.

Being employed as a United States Marshal, Edward Mars was assigned to apprehend Kate Austen after she murdered her father. After Kate purchased a ticket to Tallahassee, the Marshal arrested Kate. While driving her to prison before her arraignment, a black horse suddenly appeared in front of the car, when the Marshal swerved to avoid it he crashed the car and was momentarily unconscious. When Kate grabbed for his keys to free herself, he attempted to stop her, however; she kicked him outside of the car, and drove off leaving him on the soaking ground. (What Kate Did)

Kate later returned to Iowa to question her mother as to why she turned her in. Kate had her new friend Cassidy pose as Kate in order to throw the Marshal's off. When Edward took the glasses off of Cassidy, who was posing as a Bible saleswoman, he realized it was not Kate; but still spent the next 6 hours interrogating her. (Left Behind)

During the 3 year duration that he was chasing her, Kate began calling him at his house, and taunting him about how she was free. So, the Marshal informed her that he had her deceased friend Tom’s toy airplane in a safe deposit box in New Mexico, this lead to Kate obsessing about the object, and setting up a bank robbery to obtain it. (Whatever the Case May Be)

After she fell in love and married Kevin Callis, Kate called Edward in the hopes that she could convince him to stop chasing her. Edward agreed to stop chasing her, but only if she really settled down and stayed put, but he then added that they both know that was not going to happen. (I Do)

Finally, in Sydney Kate was finally apprehended after a tip was received from Ray Mullen who turned her in for a $23,000 reward. The Marshal and Kate boarded the plane and Kate was handcuffed to the seat, he was in seat 27G. Mid-way through the flight, a suitcase exits the overhead storage bin and hits the Marshal in the head, causing him to lose a lot of blood and go unconscious. Kate unlocks herself, places an oxygen mask on him and the back of the plane is ripped off. (Tabula Rasa)

After the crash of Oceanic Flight 815, the Marshal found himself with a large piece of shrapnel lodged in his abdomen. (Pilot - Part 1)Jack Shephard, who was the only person capable of treating him, felt it was his duty to repair the damage. The Marshal informed Jack of Kate's status as a fugitive by telling him that she was dangerous and then directing him to the mug shot located in his jacket pocket. Jack removed the piece of shrapnel and because of the lack of medicine available on the island; his only other option was to wait out his inevitable death. (Tabula Rasa)

On Day 3, the Marshal and Kate finally had their face to face talk and the Marshal expected Kate would be the one to put him out of his misery, however; it was Sawyer who would be the one to attempt a mercy killing. Sawyer regrettably missed the Marshal's heart and perforated his lung. Jack had no choice at that moment but to euthanize the Marshal. (Tabula Rasa)